A thermoplastic epoxy resin, which is a straight-chain, high-molecular-weight epoxy polymer, has recently been used for various applications because it can be melted by heating and it is superior in flexibility, processability, and adhesion property and therefore it becomes possible to allow reuse, recycle, and secondary fabrication, which have been impossible to do for conventional thermosetting epoxy resins (see, for example, Patent Documents 1 to 3).
A thermoplastic epoxy resin is a liner polymer that is formed through chain extension caused by addition polymerization of a bifunctional epoxy compound and a compound having two active hydrogen-containing groups, e.g., a bifunctional phenol compound, accompanied by ring-opening of an epoxy ring. However, since an alcoholic hydroxyl group is generated as a result of a reaction of a bifunctional epoxy compound with an active hydrogen-containing group, e.g., a phenolic hydroxyl group, a polymer with a branched chain structure is formed if the alcoholic hydroxyl group reacts on an epoxy group. Polymers having such a structure tend to become gelled and it is difficult to melt them by heating and they are poor in solubility in solvent, so that they fail to exert properties as thermoplastic resin.
Conventionally, alkali metal compounds have been known as a polymerization catalyst for polymerizing a bifunctional epoxy compound and a bifunctional phenol compound. However, there is a problem that their reaction products are prone to gelate. Therefore, what is desirable as a curing accelerator for obtaining a thermoplastic polymer by polymerizing a bifunctional epoxy compound with a bifunctional phenol compound is an agent that restrains a reaction between one epoxy group and another epoxy group or a reaction between an epoxy group and an alcoholic hydroxyl group and instead causes a reaction between an epoxy group and a phenolic hydroxyl group. Phosphorus-based compounds and imidazole-based compounds have been suggested as such a curing accelerator, and especially, phosphorus-based curing accelerators, such as triphenyl phosphine (TPP) and tri-o-tolylphosphine (TOTP), have been used from the viewpoint of realizing good thermoplasticity of a cured product (see, for example, Patent Document 3).
However, in the case of using such a curing accelerator when producing a thermoplastic cured epoxy resin by polymerizing a bifunctional epoxy compound and a bifunctional phenol compound, the following problem is known. That is, the use of such a compound causes a product to color in orange to red though the reason for this is unclear, so that the use or application of the product will be limited. This does not very often become a problem in the field of products which are less influenced by the coloring of a resin itself. Because of the coloring of a resin itself, however, conventional thermoplastic epoxy resin compositions cannot be used successfully for products with which the influence on appearance should be taken into consideration.